In numerous medical and surgical techniques, stents, grafts, or stent-grafts hereinafter sometimes referred to as implantable tubular prostheses, are inserted into body vessels or ducts or the like, temporarily or permanently, in order to repair a defect or maintain the patency of the vessel or duct lumen to as great a degree as possible. Transluminal implantation of such prostheses requires that they be introduced to the site collapsed about or within an introduction device and released to self expand or are expanded by other mechanisms to an expanded tubular state providing a lumen of approximately the same size as the patent vessel or duct lumen.
A wide variety of such tubular prostheses have been proposed for introduction through a percutaneous access site and advancement through the vascular system to an aneurysm, defect or injury site and for deployment to bridge the site. Once deployed in situ, the prosthesis must be stabilized mechanically until neointimal growth occurs over the graft ends and interior surface. Such prostheses have been formed of a wide variety of materials and shapes to accommodate particular vascular sites and to encourage neointimal growth. In addition, a wide variety of deployment mechanisms and techniques have been proposed to position and stabilize the prosthesis in place including "active" fixation mechanisms designed to penetrate the vessel wall and "passive" fixation mechanisms that press against and expand the diameter of the vessel lumen.
For example, certain expandable metal "wire stents" have been employed in clinical use for insertion into an artery in order to assist in preventing re-stenosis after a balloon angioplasty procedure has been completed to expand a stenosed site. These wire stents are relatively structurally stiff, and when expanded at the site (typically by a further balloon catheter) engage the previously (or simultaneously) compressed remnant of the occlusion in the vessel and are passively retained in place by friction until neointimal growth permanently encases the stent. Typically, such wire stents are formed of wire mesh or wire loops or are formed of a perforated metal sheet, as shown, for example, in U.S. Pat. Nos. 4,776,337, 4,877,030, 5,007,926, and 5,079,006. Such wire stents have relatively large openings in the side wall thereof in relation to the wire gauge or remaining sheet material bounding the openings. After insertion to the site of implantation, the inner diameter of the tubular wire stent can be expanded by a balloon catheter or the like into engagement with the vessel wall. The large openings result from the attempt to minimize the number of wire strands and turns of the stent, because, after expansion in situ, each exposed turn or strand slightly intrudes into the vessel lumen and causes perturbation in the blood flow and can constitute a site of formation of blood clots. In addition, the expandable wire or sheet stents must expand to achieve their objective and therefore by design must leave open spaces. The more they expand, the more open space they must leave.
Such wire stents are formed of a single layer of wire mesh or strand in either a tubular form or a rolled sheet form. When the rolled sheet form is expanded, only a minor overlap if any of adjoining edges is contemplated as shown in the structures shown in the '030, '006 and '926 patents. The resulting stent structure is porous, but blood flow through the openings between wire turns or through the perforations is not an issue because the openings or perforations bear against the vessel wall. The blood contact or circulation alongside the openings or perforations encourages neointimal ingrowth to stabilize the stent. However, the relatively large openings can also allow fibrotic build-up to occur through the openings and possibly constrict the wire stent lumen over time. In addition, the relatively large openings are not very effective in repairing other defects in the vessel wall, e.g., a tear or dissection, thus limiting the uses of such stents.
At this point, it should be pointed out that at times, the term "stent" is used interchangeably in the prior art with "graft," although vascular grafts classically are longer and have less porous side walls than the above-described wire stent. The expression "vascular graft" originally was used to described harvested blood vessels used to bypass a length of diseased or enlarged blood vessel, and the expression "artificial graft" typically connotes an elongated, biocompatible, tubular body mimicking the flexibility of the natural blood vessel it is intended to replace. In an open chest surgical procedure, the active attachment of such flexible vascular or artificial grafts to patent blood vessel ends is effected by suturing in a procedure referred to as anastomosis.
Elongated, artificial "intraluminal grafts" have also been developed for use in an intraluminal implantation procedure to bridge elongated aneurysms, defects or injuries to avoid invasive vascular surgery. Such intraluminal grafts are typically formed of a single tube of flexible biocompatible materials, e.g., a Dacron fabric tube that is long enough or shaped especially to bridge the defective region of the blood vessel, coupled with one or more retention mechanism at an end or ends thereof. Certain of the retention mechanisms proposed for use with such intraluminal grafts employ passive stents at each end that are allowed to self expand or are expanded to enlarge and frictionally engage patent vessel wall on either side of the site as shown, for example, in U.S. Pat. No. 3,991,767. The end stents may alternatively include active barbs or hooks that are manipulated to invade the patent vessel wall and retain the graft in position. Such combinations of stent and graft structures are at times referred to as "stent-grafts" and are also shown, for example, in U.S. Pat. Nos. 5,078,726 and 5,336,473.
Another artificial intraluminal stent that is typically not porous and may be used to bridge a defect or maintain the patency of an expanded vessel lumen is formed as a tubular body from a sheet of biocompatible metal, plastic or other material. The sheet metal is rolled up in a collapsed roll state in one or more overlapping layers that can be advanced intraluminally and expanded to bridge the site. Examples of such intraluminal stents and introduction systems are shown, for example, in U.S. Pat. Nos. 34,327, 4,740,207, 5,100,429, 5,306,294 and 5,405,379. Such stents are also employed as the end fixation mechanisms in stent-grafts as shown in certain embodiments of the above-referenced '726 and '473 patents.
The stents disclosed in the above-referenced '473 and '294 patents are advantageous in that they are formed of a sheet of metal foil adapted to be rolled into a plurality of sheet layers forming the collapsed and expanded rolls that are radiopaque and allow visualization of their advancement and deployment at the site. Moreover, when released at the site, no separate expansion mechanism is required because the rolled up sheet material self expands to a size that fits and slightly expands the lumen of the vessel or duct. The stent or stent-graft is retained by passive engagement of the expanded roll of sheet material against the vessel or duct lumen caused by the expansion spring force tending to further expand the diameter of the expanded tubular member. As a result of the self expansion spring force and the expansion range, sizing of the expanded tubular body diameter to the vessel or duct lumen is not critical. The multiple overlapping layers ensure that no gap can occur between the opposite edges of the sheet. Moreover, the rolled, overlapping layers forming the side wall of the stent provides substantial hoop strength to resist or prevent re-stenosis in the vessel wall. Certain of these advantages are also obtained by the nonradiopaque stent depicted in the above-referenced '579 patent.
In such self expanding (or balloon expandable) multiple layer side wall stents formed of rolled sheet material as described in the above-referenced patents, the nonporous smooth inner lumen advantageously does not create flow perturbations along its length. On the other hand, the impervious side wall formed by the roll of sheet material is likely to limit the area of neointimal growth within the stent lumen to about 0.125 inches from each end opening. Consequently, over a period of days or weeks, blood clots may be encouraged to form by contact with the exposed sheet material of the inner lumen surface. The blood clots can break away and migrate to a site posing a danger to the patient. It may therefore be necessary to suppress clot formation with anticoagulant drugs, e.g., coumarin, aspirin, or others, for extended indefinite periods of time in the case of solid sheet material stents.
Notwithstanding the foregoing, there remains a need for a stent design with sufficient hoop strength and structure for permitting neointimal tissue ingrowth, as well as the capability to treat vascular sites which may extend along curved portions of the vessel. In addition, there remains a need for grafts which can be readily implanted, are flexible to track curved portions of the vessel, and which provide sufficient support to maintain patency.